1. Field of the Invention
This invention relates to a glow plug for facilitating the ignition and the burning of an air-fuel mixture, and also a glow plug for an internal combustion engine.
2. Description of the Related Art
In recent years, more effective emission control has been demanded in spark-ignition internal combustion engines and diesel engines for the protection of environment. To meet such a demand, various proposals have been made. Examples of the proposals will be listed below. A first proposal relates to an improvement of the structure of an engine. A second proposal relates to after-treatment or post-treatment using a catalytic converter. A third proposal relates to an improvement of the properties of fuel or lubricant. A fourth proposal relates to an improvement of a burning control system for an engine.
A recent burning control system for an engine requires the detection of conditions of the burning of an air-fuel mixture in a combustion chamber of the engine. According to proposals, the pressure in a combustion chamber, the light generated by the burning of an air-fuel mixture, the ion current related to the combustion chamber, and other physical parameters are detected as an indication of conditions of the burning of the air-fuel mixture.
The detection of burning conditions in response to an ion current means a direct observation of a chemical reaction caused during the burning of an air-fuel mixture. Accordingly, it is thought that the ion-current-based detection is useful. Various methods of detecting an ion current have been proposed.
Japanese published unexamined patent application 7-259597 discloses a sensor for detecting the degree of ionization of gases in an engine combustion chamber. In Japanese application 7-259597, the sensor has a measurement sleeve electrode which is provided concentrically around a fuel injection nozzle extending into the engine combustion chamber from a cylinder head. The measurement sleeve electrode is insulated from walls of the fuel injection nozzle and walls of the cylinder head.
U.S. Pat. No. 4,739,731 discloses a ceramic glow plug designed to detect an ion current caused during the burning of an air-fuel mixture in an engine combustion chamber. In U.S. Pat. No. 4,739,731, the ceramic glow plug extends into the engine combustion chamber. A tip of the ceramic glow plug has an electrically conductive layer made of platinum. The ceramic glow plug contains an electrical conductor leading from the electrically conductive tip thereof. A direct voltage of 250 V is applied between the electrically conductive tip of the ceramic glow plug and the wall of the combustion chamber.
The sensor in Japanese application 7-259597 has the following problems. It is necessary to insulate the measurement sleeve electrode of the sensor from the walls of the fuel injection nozzle and the walls of the cylinder head. Therefore, laborious steps are required in making and locating the sensor. The measurement sleeve electrode of the sensor is expensive. As the related engine is used for a long term, carbon collects in a space between the measurement sleeve electrode and the walls of the fuel injection nozzle and a space between the measurement sleeve electrode and the walls of the cylinder head. In some cases, the measurement sleeve electrode is short-circuited to the walls of the fuel injection nozzle or the walls of the cylinder head by the collected carbon.
The ceramic glow plug of U.S. Pat. No. 4,739,731 has the following problem. A large amount of platinum is used in making the ceramic glow plug. Therefore, the ceramic glow plug is expensive.